The standard message coding format for the national paging system of the British Post Office, and which is widely used by many radio common carrier (RCC) service providers throughout the world is shown in FIG. 1A, and is commonly known as the POCSAG signaling protocol, or format. Each transmission consists of a preamble portion 102 followed by one or more batches 104 and 106 of selectively arranged code words. The preamble 102 is a series of logic states (either 1's and 0's) which has a total duration of at least 576 bits (shown in 108). The preamble 102 permits the pagers receiving the preamble to attain bit synchronization and also helps the pagers acquire word synchronization. A code word 110 is a 32 bit data sequence which contains information either for synchronization or for addressing an individual pager. Each batch 104 and 106 begins with a 32 bit synchronization code word 114 which is an invariant predetermined sequence. A batch 112 comprises one synchronization code word 114 and sixteen address or data code words which are subdivided into eight separate frames each containing two address code words, or an address code word and a data code word, or two data code words. Subsequent batches in any message transmission contain the same format of a 32 bit synchronization word followed by eight frames, each containing two address code words, or an address code word and a data word, or two data words, for a total of 16 address and/or data code words in a batch.
Code words 110, either address or data, are grouped into eight frames numbered 0 to 7. The entire pager address population is similarly divided into eight possible groups, thereby providing each group with a battery saving capability. Each pager address is allocated to one of the corresponding eight frames according to the three least significant bits of its 21 bit code word identity. Thus, all pagers having addresses which end in 000 would be positioned in frame 0, and similarly, all pagers having an address code word ending in 111 would be in frame 7.
Address code words 110 have a 32 bit structure and bit 1 of an address code word is always zero. Bits 2-19 are address bits corresponding to the 18 most significant bits of the 21 bit identity sequence assigned to an individual pager. The three least significant bits are not transmitted since they merely serve to define the frame in which the address code word must be transmitted and are therefore derivable from the frame position in a batch. Bits 20 and 21 are function bits which are used to select the required addresses from the four possible addresses assigned to the pager. Bits 22 to 31 are parity check bits and the final bit, bit 32, is chosen to give even parity. The decoding system will only examine address code words in a designated frame, and therefore, each pager's address code words is only transmitted in the frame that is allocated to that code word format.
A data code word also has a 32 bit structure, however, bit 1 of the data code word is always 1, identifying the code word as a data code word. Bits 2-21 are the data bits corresponding to the message digits, as in a numeric page, or the message characters, as in an alphanumeric message. Five 4-bit BCD numeric digits, or almost three 7-bit ASCII alphanumeric characters can be transmitted in the data bits of each data code word. Bits 22 to 31 are parity check bits and the final bit, bit 32, is chosen to give even parity, as for the address code word.
The 21 bit binary sequence for pager identity in the POCSAG signalling protocol allows for over two million combinations of possible addresses for pagers. The addition of the two function bits provides a total of eight million message combinations comprising batch signalling functions and pager addresses.
In the absence of an address codeword or message codeword, an idle codeword is transmitted. The idle codeword is a valid address codeword, which is not allocated to pagers and has the structure shown in FIG. 1B.
The POCSAG signalling protocol has been widely used in local area paging systems, as well as have other signalling protocols (GOLAY, etc.), to provide local message delivery to pagers assigned to operate within the local area paging system. Because of the address capacity provided by POCSAG signalling protocol, and other digital signalling protocols, local area systems providing wide area paging system coverage to large metropolitan areas encompassing many towns and cities have become feasible. While the local area paging systems as described have fulfilled the needs of local paging users, especially in the larger metropolitan areas, the needs of users that move from one paging system to another paging system have not been met.
With the advent of synthesized pagers having the capabilities to scan or manually change frequencies, codes, coding formats and options which are configured to the different frequencies, Radio Common Carriers (RCC's) are searching for ways to increase revenues by offering multiple coverage areas (regional, national, or international) with one pager. For example, a businessman who travels between two metropolitan areas (e.g. New York and Chicago) that have local paging systems having different operating parameters, requires two pagers to stay in constant contact, or must use the more expensive nationwide paging services that are available. An example of a pager which can switch between a nation wide paging system and a local paging system is disclosed in U.S. Pat. No. 4,644,347 by Lucas, et al. entitled "Multiple Frequency Message System".
A problem arises with the prior art when a user needs to operate in a plurality of independent paging systems. A user that is constantly changing locations amongst a plurality of different independent paging systems requires someway of being able to receive paging messages without having to either manually switch systems or having in anyway coordinate his present location amongst the paging systems. A need therefore exists, for a pager which can automatically operate in multiple independent paging systems without requiring any intervention by the pager user.